In-line optical fiber refractive index "gratings" are known. See, for instance, U.S. Pat. No. 4,807,950, incorporated herein by reference. See also U.S. Pat. No. 4,725,110, which claims a method of producing such a grating. The currently preferred method of writing an in-line grating into optical fiber comprises interference between two beams of actinic (typically UV) radiation in a portion of the fiber. The two beams are incident on the fiber in a transverse direction, the angle between the two beams (and the wavelength of the radiation) defining the grating spacing.
Typically fiber gratings are formed in Ge-doped fiber. See, for instance, F. Ouellette et al., Applied Physics Letters, Vol. 58(17), p. 1813, which inter alia discloses that the sensitivity of the Ge-doped fiber to actinic radiation can be enhanced by a thermal hydrogen treatment (4 hours at 400.degree. C. in 12 atm of H.sub.2). See also U.S. Pat. No. 5,157,747, which discloses a process of manufacturing Ge-doped fiber that enhances the GeO/GeO.sub.2 ratio in the core of the fiber, thereby enhancing the sensitivity of the fiber to actinic radiation. The process involves, exemplarily, collapsing the perform tube in a substantially oxygen-free atmosphere.
In-line optical fiber refractive index gratings can advantageously be incorporated into, for instance, optical fiber communication systems. An exemplary application of such a grating is as a reflector of pump radiation in optical fiber amplifiers, e.g., Er-doped fiber amplifiers. Since currently used pump radiation sources typically emit over a relatively wide spectral region (e.g., &gt;10 nm), it would be desirable to have available in-line fiber reflectors that have a relatively wide bandwidth, preferably, but not necessarily, as wide as the bandwidth of the pump source. Other exemplary potential applications of a relatively wide bandwidth in-line optical fiber grating are for flattening the gain curve of an optical fiber amplifier (e.g., an Er-doped fiber amplifier) and as a wide-band transmission filter, analogous to the narrow-band filter proposed by K.O. Hill et al., Electronics Letters, Vol. 23, p. 465 (1987).
International Patent application PCT/GB91/00797 (publication number WO 91/18434) discloses a broadband fiber grating that utilizes periodic fiber deformation to achieve periodic variation of the propagation constant.
Despite the clear need for optical fiber index gratings that have relatively large bandwidth, practitioners have up to now failed to produce such gratings. For instance, G. Meltz et al, SPIE, Vol. 1516, International Workshop on Photoinduced Self-Organization in Optical Fiber; Quebec City, Canada, May 10-11, 1991, report a "broadband" grating that had a transmission bandwidth of only 1.1 nm. The minimum transmissivity of the grating was about 35%, implying a peak reflectivity of only about 65%. See FIG. 4 of the publication. The peak index change in the grating was reported to be 5.5.times.10.sup.-4. E. Fertein et at., Electronics Letters, Vol. 27(20), pages 1838-1839, disclose a grating in GeO.sub.2 doped silica fiber that allegedly had substantially zero transmittance, but a FWHM (full width at half maximum) bandwidth of only 0.7 nm.
This application discloses strong, wide bandwidth in-line optical waveguide gratings that can advantageously be used in such applications as optical fiber communications or fiber lasers.